Tutorial: HTTP/2 server

In this tutorial, we are going to write a single-threaded, event-based HTTP/2 web server, which supports HTTPS only. It can handle concurrent multiple requests, but only the GET method is supported. The complete source code, libevent-server.c_, is attached at the end of this page. The source also resides in the examples directory in the archive or repository.

This simple server takes 3 arguments: The port number to listen on, the path to your SSL/TLS private key file, and the path to your certificate file. The synopsis is:

.. code-block:: text

$ libevent-server PORT /path/to/server.key /path/to/server.crt

We use libevent in this tutorial to handle networking I/O. Please note that nghttp2 itself does not depend on libevent.

The server starts with some libevent and OpenSSL setup in the main() and run() functions. This setup isn't specific to nghttp2, but one thing you should look at is setup of ALPN callback. The ALPN callback is used by the server to select application protocols offered by client. In ALPN, client sends the list of supported application protocols, and server selects one of them. We provide the callback for it::

static int alpn_select_proto_cb(SSL *ssl _U_, const unsigned char **out,
                                unsigned char *outlen, const unsigned char *in,
                                unsigned int inlen, void *arg _U_) {
  int rv;

  rv = nghttp2_select_alpn(out, outlen, in, inlen);

  if (rv != 1) {
    return SSL_TLSEXT_ERR_NOACK;
  }

  return SSL_TLSEXT_ERR_OK;
}

static SSL_CTX *create_ssl_ctx(const char *key_file, const char *cert_file) {
  SSL_CTX *ssl_ctx;
  EC_KEY *ecdh;

  ssl_ctx = SSL_CTX_new(SSLv23_server_method());

  ...

  SSL_CTX_set_alpn_select_cb(ssl_ctx, alpn_select_proto_cb, NULL);

  return ssl_ctx;
}

In alpn_select_proto_cb(), we use nghttp2_select_alpn() to select application protocol. The nghttp2_select_alpn() returns 1 only if it selected h2 (ALPN identifier for HTTP/2), and out parameters were assigned accordingly.

Next, let's take a look at the main structures used by the example application:

We use the app_context structure to store application-wide data::

struct app_context {
  SSL_CTX *ssl_ctx;
  struct event_base *evbase;
};

We use the http2_session_data structure to store session-level (which corresponds to one HTTP/2 connection) data::

typedef struct http2_session_data {
  struct http2_stream_data root;
  struct bufferevent *bev;
  app_context *app_ctx;
  nghttp2_session *session;
  char *client_addr;
} http2_session_data;

We use the http2_stream_data structure to store stream-level data::

typedef struct http2_stream_data {
  struct http2_stream_data *prev, *next;
  char *request_path;
  int32_t stream_id;
  int fd;
} http2_stream_data;

A single HTTP/2 session can have multiple streams. To manage them, we use a doubly linked list: The first element of this list is pointed to by the root->next in http2_session_data. Initially, root->next is NULL.

libevent's bufferevent structure is used to perform network I/O, with the pointer to the bufferevent stored in the http2_session_data structure. Note that the bufferevent object is kept in http2_session_data and not in http2_stream_data. This is because http2_stream_data is just a logical stream multiplexed over the single connection managed by the bufferevent in http2_session_data.

We first create a listener object to accept incoming connections. libevent's struct evconnlistener is used for this purpose::

static void start_listen(struct event_base *evbase, const char *service,
                         app_context *app_ctx) {
  int rv;
  struct addrinfo hints;
  struct addrinfo *res, *rp;

  memset(&hints, 0, sizeof(hints));
  hints.ai_family = AF_UNSPEC;
  hints.ai_socktype = SOCK_STREAM;
  hints.ai_flags = AI_PASSIVE;
#ifdef AI_ADDRCONFIG
  hints.ai_flags |= AI_ADDRCONFIG;
#endif /* AI_ADDRCONFIG */

  rv = getaddrinfo(NULL, service, &hints, &res);
  if (rv != 0) {
    errx(1, NULL);
  }
  for (rp = res; rp; rp = rp->ai_next) {
    struct evconnlistener *listener;
    listener = evconnlistener_new_bind(
        evbase, acceptcb, app_ctx, LEV_OPT_CLOSE_ON_FREE | LEV_OPT_REUSEABLE,
        16, rp->ai_addr, (int)rp->ai_addrlen);
    if (listener) {
      freeaddrinfo(res);

      return;
    }
  }
  errx(1, "Could not start listener");
}

We specify the acceptcb callback, which is called when a new connection is accepted::

static void acceptcb(struct evconnlistener *listener _U_, int fd,
                     struct sockaddr *addr, int addrlen, void *arg) {
  app_context *app_ctx = (app_context *)arg;
  http2_session_data *session_data;

  session_data = create_http2_session_data(app_ctx, fd, addr, addrlen);

  bufferevent_setcb(session_data->bev, readcb, writecb, eventcb, session_data);
}

Here we create the http2_session_data object. The connection's bufferevent is initialized at the same time. We specify three callbacks for the bufferevent: readcb, writecb, and eventcb.

The eventcb() callback is invoked by the libevent event loop when an event (e.g. connection has been established, timeout, etc.) occurs on the underlying network socket::

static void eventcb(struct bufferevent *bev _U_, short events, void *ptr) {
  http2_session_data *session_data = (http2_session_data *)ptr;
  if (events & BEV_EVENT_CONNECTED) {
    const unsigned char *alpn = NULL;
    unsigned int alpnlen = 0;
    SSL *ssl;

    fprintf(stderr, "%s connected\n", session_data->client_addr);

    ssl = bufferevent_openssl_get_ssl(session_data->bev);

    SSL_get0_alpn_selected(ssl, &alpn, &alpnlen);

    if (alpn == NULL || alpnlen != 2 || memcmp("h2", alpn, 2) != 0) {
      fprintf(stderr, "%s h2 is not negotiated\n", session_data->client_addr);
      delete_http2_session_data(session_data);
      return;
    }

    initialize_nghttp2_session(session_data);

    if (send_server_connection_header(session_data) != 0 ||
        session_send(session_data) != 0) {
      delete_http2_session_data(session_data);
      return;
    }

    return;
  }
  if (events & BEV_EVENT_EOF) {
    fprintf(stderr, "%s EOF\n", session_data->client_addr);
  } else if (events & BEV_EVENT_ERROR) {
    fprintf(stderr, "%s network error\n", session_data->client_addr);
  } else if (events & BEV_EVENT_TIMEOUT) {
    fprintf(stderr, "%s timeout\n", session_data->client_addr);
  }
  delete_http2_session_data(session_data);
}

Here we validate that HTTP/2 is negotiated, and if not, drop connection.

For the BEV_EVENT_EOF, BEV_EVENT_ERROR, and BEV_EVENT_TIMEOUT events, we just simply tear down the connection. The delete_http2_session_data() function destroys the http2_session_data object and its associated bufferevent member. As a result, the underlying connection is closed.

The BEV_EVENT_CONNECTED event is invoked when SSL/TLS handshake has completed successfully. After this we are ready to begin communicating via HTTP/2.

The initialize_nghttp2_session() function initializes the nghttp2 session object and several callbacks::

static void initialize_nghttp2_session(http2_session_data *session_data) {
  nghttp2_session_callbacks *callbacks;

  nghttp2_session_callbacks_new(&callbacks);

  nghttp2_session_callbacks_set_send_callback2(callbacks, send_callback);

  nghttp2_session_callbacks_set_on_frame_recv_callback(callbacks,
                                                       on_frame_recv_callback);

  nghttp2_session_callbacks_set_on_stream_close_callback(
      callbacks, on_stream_close_callback);

  nghttp2_session_callbacks_set_on_header_callback(callbacks,
                                                   on_header_callback);

  nghttp2_session_callbacks_set_on_begin_headers_callback(
      callbacks, on_begin_headers_callback);

  nghttp2_session_server_new(&session_data->session, callbacks, session_data);

  nghttp2_session_callbacks_del(callbacks);
}

Since we are creating a server, we use nghttp2_session_server_new() to initialize the nghttp2 session object. We also setup 5 callbacks for the nghttp2 session, these are explained later.

The server now begins by sending the server connection preface, which always consists of a SETTINGS frame. send_server_connection_header() configures and submits it::

static int send_server_connection_header(http2_session_data *session_data) {
  nghttp2_settings_entry iv[1] = {
      {NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS, 100}};
  int rv;

  rv = nghttp2_submit_settings(session_data->session, NGHTTP2_FLAG_NONE, iv,
                               ARRLEN(iv));
  if (rv != 0) {
    warnx("Fatal error: %s", nghttp2_strerror(rv));
    return -1;
  }
  return 0;
}

In the example SETTINGS frame we've set SETTINGS_MAX_CONCURRENT_STREAMS to 100. nghttp2_submit_settings() is used to queue the frame for transmission, but note it only queues the frame for transmission, and doesn't actually send it. All functions in the nghttp2_submit_*() family have this property. To actually send the frame, nghttp2_session_send() should be used, as described later.

Since bufferevent may buffer more than the first 24 bytes from the client, we have to process them here since libevent won't invoke callback functions for this pending data. To process the received data, we call the session_recv() function::

static int session_recv(http2_session_data *session_data) {
  nghttp2_ssize readlen;
  struct evbuffer *input = bufferevent_get_input(session_data->bev);
  size_t datalen = evbuffer_get_length(input);
  unsigned char *data = evbuffer_pullup(input, -1);

  readlen = nghttp2_session_mem_recv2(session_data->session, data, datalen);
  if (readlen < 0) {
    warnx("Fatal error: %s", nghttp2_strerror((int)readlen));
    return -1;
  }
  if (evbuffer_drain(input, (size_t)readlen) != 0) {
    warnx("Fatal error: evbuffer_drain failed");
    return -1;
  }
  if (session_send(session_data) != 0) {
    return -1;
  }
  return 0;
}

In this function, we feed all unprocessed but already received data to the nghttp2 session object using the nghttp2_session_mem_recv2() function. The nghttp2_session_mem_recv2() function processes the data and may both invoke the previously setup callbacks and also queue outgoing frames. To send any pending outgoing frames, we immediately call session_send().

The session_send() function is defined as follows::

static int session_send(http2_session_data *session_data) {
  int rv;
  rv = nghttp2_session_send(session_data->session);
  if (rv != 0) {
    warnx("Fatal error: %s", nghttp2_strerror(rv));
    return -1;
  }
  return 0;
}

The nghttp2_session_send() function serializes the frame into wire format and calls the send_callback(), which is of type :type:nghttp2_send_callback2. The send_callback() is defined as follows::

static nghttp2_ssize send_callback(nghttp2_session *session _U_,
                                   const uint8_t *data, size_t length,
                                   int flags _U_, void *user_data) {
  http2_session_data *session_data = (http2_session_data *)user_data;
  struct bufferevent *bev = session_data->bev;
  /* Avoid excessive buffering in server side. */
  if (evbuffer_get_length(bufferevent_get_output(session_data->bev)) >=
      OUTPUT_WOULDBLOCK_THRESHOLD) {
    return NGHTTP2_ERR_WOULDBLOCK;
  }
  bufferevent_write(bev, data, length);
  return (nghttp2_ssize)length;
}

Since we use bufferevent to abstract network I/O, we just write the data to the bufferevent object. Note that nghttp2_session_send() continues to write all frames queued so far. If we were writing the data to a non-blocking socket directly using the write() system call in the send_callback(), we'd soon receive an EAGAIN or EWOULDBLOCK error since sockets have a limited send buffer. If that happens, it's possible to return :macro:NGHTTP2_ERR_WOULDBLOCK to signal the nghttp2 library to stop sending further data. But here, when writing to the bufferevent, we have to regulate the amount data to buffered ourselves to avoid using huge amounts of memory. To achieve this, we check the size of the output buffer and if it reaches more than or equal to OUTPUT_WOULDBLOCK_THRESHOLD bytes, we stop writing data and return :macro:NGHTTP2_ERR_WOULDBLOCK.

The next bufferevent callback is readcb(), which is invoked when data is available to read in the bufferevent input buffer::

static void readcb(struct bufferevent *bev _U_, void *ptr) {
  http2_session_data *session_data = (http2_session_data *)ptr;
  if (session_recv(session_data) != 0) {
    delete_http2_session_data(session_data);
    return;
  }
}

In this function, we just call session_recv() to process incoming data.

The third bufferevent callback is writecb(), which is invoked when all data in the bufferevent output buffer has been sent::

static void writecb(struct bufferevent *bev, void *ptr) {
  http2_session_data *session_data = (http2_session_data *)ptr;
  if (evbuffer_get_length(bufferevent_get_output(bev)) > 0) {
    return;
  }
  if (nghttp2_session_want_read(session_data->session) == 0 &&
      nghttp2_session_want_write(session_data->session) == 0) {
    delete_http2_session_data(session_data);
    return;
  }
  if (session_send(session_data) != 0) {
    delete_http2_session_data(session_data);
    return;
  }
}

First we check whether we should drop the connection or not. The nghttp2 session object keeps track of reception and transmission of GOAWAY frames and other error conditions as well. Using this information, the nghttp2 session object can state whether the connection should be dropped or not. More specifically, if both nghttp2_session_want_read() and nghttp2_session_want_write() return 0, the connection is no-longer required and can be closed. Since we are using bufferevent and its deferred callback option, the bufferevent output buffer may still contain pending data when the writecb() is called. To handle this, we check whether the output buffer is empty or not. If all of these conditions are met, we drop connection.

Otherwise, we call session_send() to process the pending output data. Remember that in send_callback(), we must not write all data to bufferevent to avoid excessive buffering. We continue processing pending data when the output buffer becomes empty.

We have already described the nghttp2 callback send_callback(). Let's learn about the remaining nghttp2 callbacks setup in initialize_nghttp2_setup() function.

The on_begin_headers_callback() function is invoked when the reception of a header block in HEADERS or PUSH_PROMISE frame is started::

static int on_begin_headers_callback(nghttp2_session *session,
                                     const nghttp2_frame *frame,
                                     void *user_data) {
  http2_session_data *session_data = (http2_session_data *)user_data;
  http2_stream_data *stream_data;

  if (frame->hd.type != NGHTTP2_HEADERS ||
      frame->headers.cat != NGHTTP2_HCAT_REQUEST) {
    return 0;
  }
  stream_data = create_http2_stream_data(session_data, frame->hd.stream_id);
  nghttp2_session_set_stream_user_data(session, frame->hd.stream_id,
                                       stream_data);
  return 0;
}

We are only interested in the HEADERS frame in this function. Since the HEADERS frame has several roles in the HTTP/2 protocol, we check that it is a request HEADERS, which opens new stream. If the frame is a request HEADERS, we create a http2_stream_data object to store the stream related data. We associate the created http2_stream_data object with the stream in the nghttp2 session object using nghttp2_set_stream_user_data(). The http2_stream_data object can later be easily retrieved from the stream, without searching through the doubly linked list.

In this example server, we want to serve files relative to the current working directory in which the program was invoked. Each header name/value pair is emitted via on_header_callback function, which is called after on_begin_headers_callback()::

static int on_header_callback(nghttp2_session *session,
                              const nghttp2_frame *frame, const uint8_t *name,
                              size_t namelen, const uint8_t *value,
                              size_t valuelen, uint8_t flags _U_,
                              void *user_data _U_) {
  http2_stream_data *stream_data;
  const char PATH[] = ":path";
  switch (frame->hd.type) {
  case NGHTTP2_HEADERS:
    if (frame->headers.cat != NGHTTP2_HCAT_REQUEST) {
      break;
    }
    stream_data =
        nghttp2_session_get_stream_user_data(session, frame->hd.stream_id);
    if (!stream_data || stream_data->request_path) {
      break;
    }
    if (namelen == sizeof(PATH) - 1 && memcmp(PATH, name, namelen) == 0) {
      size_t j;
      for (j = 0; j < valuelen && value[j] != '?'; ++j)
        ;
      stream_data->request_path = percent_decode(value, j);
    }
    break;
  }
  return 0;
}

We search for the :path header field among the request headers and store the requested path in the http2_stream_data object. In this example program, we ignore the :method header field and always treat the request as a GET request.

The on_frame_recv_callback() function is invoked when a frame is fully received::

static int on_frame_recv_callback(nghttp2_session *session,
                                  const nghttp2_frame *frame, void *user_data) {
  http2_session_data *session_data = (http2_session_data *)user_data;
  http2_stream_data *stream_data;
  switch (frame->hd.type) {
  case NGHTTP2_DATA:
  case NGHTTP2_HEADERS:
    /* Check that the client request has finished */
    if (frame->hd.flags & NGHTTP2_FLAG_END_STREAM) {
      stream_data =
          nghttp2_session_get_stream_user_data(session, frame->hd.stream_id);
      /* For DATA and HEADERS frame, this callback may be called after
         on_stream_close_callback. Check that stream still alive. */
      if (!stream_data) {
        return 0;
      }
      return on_request_recv(session, session_data, stream_data);
    }
    break;
  default:
    break;
  }
  return 0;
}

First we retrieve the http2_stream_data object associated with the stream in on_begin_headers_callback() using nghttp2_session_get_stream_user_data(). If the requested path cannot be served for some reason (e.g. file is not found), we send a 404 response using error_reply(). Otherwise, we open the requested file and send its content. We send the header field :status as a single response header.

Sending the file content is performed by the send_response() function::

static int send_response(nghttp2_session *session, int32_t stream_id,
                         nghttp2_nv *nva, size_t nvlen, int fd) {
  int rv;
  nghttp2_data_provider2 data_prd;
  data_prd.source.fd = fd;
  data_prd.read_callback = file_read_callback;

  rv = nghttp2_submit_response2(session, stream_id, nva, nvlen, &data_prd);
  if (rv != 0) {
    warnx("Fatal error: %s", nghttp2_strerror(rv));
    return -1;
  }
  return 0;
}

nghttp2 uses the :type:nghttp2_data_provider2 structure to send the entity body to the remote peer. The source member of this structure is a union, which can be either a void pointer or an int (which is intended to be used as file descriptor). In this example server, we use it as a file descriptor. We also set the file_read_callback() callback function to read the contents of the file::

static nghttp2_ssize file_read_callback(nghttp2_session *session _U_,
                                        int32_t stream_id _U_, uint8_t *buf,
                                        size_t length, uint32_t *data_flags,
                                        nghttp2_data_source *source,
                                        void *user_data _U_) {
  int fd = source->fd;
  ssize_t r;
  while ((r = read(fd, buf, length)) == -1 && errno == EINTR)
    ;
  if (r == -1) {
    return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
  }
  if (r == 0) {
    *data_flags |= NGHTTP2_DATA_FLAG_EOF;
  }
  return (nghttp2_ssize)r;
}

If an error occurs while reading the file, we return :macro:NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE. This tells the library to send RST_STREAM to the stream. When all data has been read, the :macro:NGHTTP2_DATA_FLAG_EOF flag is set to signal nghttp2 that we have finished reading the file.

The nghttp2_submit_response2() function is used to send the response to the remote peer.

The on_stream_close_callback() function is invoked when the stream is about to close::

static int on_stream_close_callback(nghttp2_session *session, int32_t stream_id,
                                    uint32_t error_code _U_, void *user_data) {
  http2_session_data *session_data = (http2_session_data *)user_data;
  http2_stream_data *stream_data;

  stream_data = nghttp2_session_get_stream_user_data(session, stream_id);
  if (!stream_data) {
    return 0;
  }
  remove_stream(session_data, stream_data);
  delete_http2_stream_data(stream_data);
  return 0;
}

Lastly, we destroy the http2_stream_data object in this function, since the stream is about to close and we no longer need the object.